Gas Burner Won't Light: Is It Gas, Air or Ignition?

“It won’t fire — must be the gas valve.” It is the most reflexive call in a commercial boiler room, and the most expensive one when it turns out to be wrong. A gas burner won’t light the same way whether the root cause sits in the gas train, in the air side or in the ignition circuit: nothing happens, or the unit tries and locks out. The three families look identical from across the room and separate cleanly under a meter. Done in order, the triage takes about fifteen minutes. Done on instinct, it takes two truck rolls and a part you did not need.

Gas burner won’t light: three families, one symptom

A commercial burner does not fire because someone asked for heat. It fires because it has satisfied, in order, a list of conditions the control checks one at a time. Three families can break that chain:

• Air — combustion air intake, vent, blower, air proving switch. Until air is proven, the control opens nothing.
• Gas — supply pressure, high and low gas pressure switches, gas train. Fuel can be present at the shut-off and absent at the burner.
• Ignition and detection — spark or hot surface igniter, electrode position and gap, ground path, flame signal. Here, flame can exist and still not count.

The fourth family — waterside safeties — is out of scope here. When the unit fires and then drops out cycle after cycle, that is a recurring lockout and it has its own method.

Tools required

What the control already logged for you

Before a panel comes off, the ignition control has often done half the work. The Fenwal Series 35-60 direct spark control data sheet defines three LED states: steady on for an internal control failure, two flashes for flame sensed with no call for heat, three flashes for ignition lockout. Two flashes and three flashes send a technician to two entirely different jobs.

The same sheet lists numbers you cannot guess from the field: trial-for-ignition periods of 4.0, 7.0, 10.0 or 15.0 seconds depending on version, and prepurge of none, 15 or 30 seconds. The control may also reset itself automatically one hour after a lockout — which is how a permanent fault gets logged as “intermittent.”

Where the sequence stops names the family. That is the whole triage:

Ruling out air: a running blower proves nothing

This is the most persistent confusion in the trade. A spinning blower does not establish air proving — the pressure switch does, and it can refuse to make while the motor sounds perfect. Three causes dominate: a partly blocked intake or vent, a fouled or kinked sensing tube, and a tired contact that no longer transfers at setpoint.

So the useful check is not “is it blowing,” it is “does the contact transfer when it should.” On atmospheric appliances, add a draft check: a shared flue or a cold chimney creates an air-side fault that has nothing to do with the appliance itself.

Ruling out gas: pressure means nothing at rest

A reading taken with the burner off proves nothing. The measurement that counts is taken with the appliance running at maximum demand and compared to the rating plate — never to a value remembered from another site. Supply that holds at idle and collapses under load points upstream: regulator, plugged filter, undersized piping, or three appliances starting together on one branch. The full procedure is in our guide to gas pressure setup.

In Québec these installations fall under CSA B149.1, adopted by reference into the Gas chapter of the Construction Code and administered by the RBQ. Any work on a gas train — including opening a pressure tap — requires the matching licence.

Ruling out ignition: spark first, signal second

Two distinct steps that haste collapses into one. Spark: present or absent, crisp or weak, in the right place. It depends on the high-tension lead, the transformer, the ground path and the position of the electrode assembly. The 35-60 sheet places the tips inside the flame envelope, about 1/2 in (1 cm) above the base of the flame, with a 0.125 ± 0.031 in (3.12 ± 0.81 mm) gap — and states that electrodes are not field adjustable: out of tolerance, the assembly gets replaced.

Flame signal second. It is measured in DC microamps at the manufacturer’s test points. On this control the minimum required to stay out of lockout is 0.7 µA, read on a DC microammeter across the FC− / FC+ pins; a negative reading just means the leads are reversed. The Platform Ignition Module in the same family scales its test output to 1 µA per volt, so flame current can be read directly on the volts scale of an ordinary digital meter.

The rule that carries across all equipment: the manufacturer’s threshold governs, not a shop habit. A signal that clears the minimum but wanders at start-up is the same failure, a few weeks early.

⚠ Safety

Every trial for ignition delivers fuel into the chamber before flame is established — do not cycle a burner repeatedly “to see.” De-energize and lock out before touching wiring, and only open a pressure tap if you hold the required licence.

The usual innocent suspects

• The gas valve — blamed on reflex, when in most cases it never received its open signal. Check coil voltage before ordering anything.
• The electrode that “looks fine” — a visual check says nothing about actual gap or ground quality. Only the signal reading settles it.
• A brand-new air pressure switch — swapped because it “won’t make,” when the tube was plugged or the intake iced over. The new one won’t make either.
• The cold — a properly installed appliance starts at −25 °C. Cold does not cause the failure; it exposes one that was already there.

Field case: the new gas valve that changed nothing

Light industrial building in Laval, hot water boiler with a forced draught burner, two service calls in ten days. First visit: lockout confirmed, gas valve declared faulty, part ordered. Second visit: valve replaced, same lockout, same minute into the cycle.

Picking the file back up started with the only useful question: where does the sequence stop? Answer — the blower started, but no spark ever snapped. The control never began its trial for ignition. The air pressure switch sensing tube, packed with shop dust, kept the contact from transferring. Tube cleaned, contact verified at setpoint, normal start. Real cost of the shortcut: one gas valve, two truck rolls and nine days without heat in a warehouse — over a 6 mm tube. Our service calls across Laval now open with that question, before anything comes apart.

Why does this triage fail most often in January?

Because a Montréal winter adds causes the other seasons hide. Sidewall combustion air intakes on basement boiler rooms pack with powder snow and glaze over after a storm followed by a thaw — a pattern that repeats several times each winter across Greater Montréal. Condensing appliance vents ice up at the termination. Flat commercial roofs drift snow around vent terminations. The result: the air family dominates January calls, while the ignition family shows up in the fall restart, after months of standing idle.

Winter urgency also pushes crews to skip the triage: it’s cold, tenants are calling, so the most likely part gets changed. That is exactly where a written method pays — it holds up under pressure.

Fifteen minutes before you order a part

Next time a unit refuses to start, impose one rule: no part is ordered until three things are on the service ticket — the control’s fault code, the exact point where the sequence stops, and the measured value of the matching discriminating test. Three lines, and a service call stops being a hopeful parts swap.

What a building manager gains from it is not technical: they pay for an identified cause instead of an installed guess, and the second truck roll simply never happens. It is the method Montréal Combustion technicians bring to their gas heating and combustion work — measure before dismantling, and be able to say why the part going in is the right one.